1. Field of the Invention
This invention relates to an electrical turbocompound machine for an internal combustion engine. More particularly, this invention relates to an air cooling system for cooling an electric motor within a turbocharger.
2. Description of Related Art
A turbocharger is a type of forced induction system used with internal combustion engines. Turbochargers deliver compressed air to an engine intake, allowing more fuel to be combusted, thus boosting an engine's power density without significantly increasing engine weight. Thus, turbochargers permit the use of smaller engines that develop the same amount of power as larger, normally aspirated engines. Using a smaller engine in a vehicle has the desired effect of decreasing the mass of the vehicle, increasing performance, and enhancing fuel economy. Moreover, the use of turbochargers permits more complete combustion of the fuel delivered to the engine, which contributes to the highly desirable goal of reduced emissions.
Turbochargers include a turbine stage having a turbine housing connected to the engine's exhaust manifold, a compressor stage having a compressor housing connected to the engine's intake manifold, and a bearing housing connecting the turbine and compressor housings together. The turbine stage includes a turbine wheel disposed within the turbine housing and the compressor stage includes a compressor impeller disposed within the compressor housing. The turbine wheel is rotatably driven by an inflow of exhaust gas supplied from the exhaust manifold. A shaft is rotatably supported in the bearing housing and couples the turbine wheel to the compressor impeller such that rotation of the turbine wheel causes rotation of the compressor impeller. The shaft connecting the turbine wheel and the compressor impeller defines an axis of rotation. As the compressor impeller rotates, it compresses ambient air entering the compressor housing, thereby increasing the air mass flow rate, airflow density, and air pressure delivered to the engine's cylinders via the engine's intake manifold.
At low engine speeds, exhaust gas is supplied to the turbine wheel at a lower pressure causing the turbine wheel and compressor impeller to rotate slower, resulting in the air entering the compressor housing being compressed less, which results in a so-called “turbo-lag.” In order to minimize turbo-lag and improve turbocharger efficiency, and therefore engine efficiency, it is known to incorporate an electric motor into the turbocharger. This type of turbocharger is commonly referred to as an electrical turbocompound machine or electrically assisted turbocharger. The electric motor is energized at low engine speeds to impart additional torque to the shaft of the turbocharger, which causes the turbine wheel and compressor impeller to rotate faster, increasing the air mass flow rate delivered to the engine than would otherwise be delivered by a non-electrically assisted turbocharger. The electric motor can also be used as a generator, which converts shaft work, i.e., rotation of the shaft, into electrical power. The electrical power produced by the generator can be used to run auxiliary electrical components or to augment engine power.
One example of an electric motor that is incorporated into the turbocharger is a switched reluctance motor (SRM). The principles of operation of SRMs are simple, well known, and based on reluctance torque. SRMs have a stator with concentrated windings and a rotor with no winding. In a typical electrically assisted turbocharger, the SRM is located in a motor chamber defined within the bearing housing. The rotor is integrated with or mounted on the shaft and is positioned generally between a set of shaft bearings. The stator is fixedly secured and surrounds the rotor. A typical SRM may have six stator poles and four rotor poles, denoted as a “6/4 SRM.” The 6/4 SRM has three phases, each phase consisting of two windings on opposite stator poles. The windings in one phase are simultaneously energized and generate a magnetic flux. The magnetic flux created by the windings follows the path of least magnetic reluctance, meaning the flux will flow through the rotor poles that are closest to the energized stator poles, thereby magnetizing those rotor poles and causing the rotor to align itself with the energized stator poles. Electromagnetic torque is produced by the tendency of the rotor poles to align with the energized stator poles. As the rotor turns, different phases will be sequentially energized to keep the rotor turning. For use as a generator, the phases are energized when the stator poles and rotor poles are separating, rather than when they are approaching.
A liquid cooling system is typically provided as a primary means of minimizing heat transfer from the exhaust gas in the turbine stage to the electric motor in the bearing housing. However, under certain operating conditions, it is recognized that the liquid cooling system may not adequately cool the electric motor. As such, it is desirable to supplement the liquid cooling system with an air cooling system.
Air cooling systems for electrically assisted turbochargers are generally well known. For example, U.S. Pat. No. 5,605,045 discloses an electrically assisted turbocharger 10 including a shaft 16 having a turbine wheel 20 mounted on one end and a compressor impeller 28 mounted on the opposite end. The turbocharger 10 also includes an electric motor 56 housed within a bearing housing 30. An annular oil passage 38 in the bearing housing 30 is directly outside the electric motor 56 and it is the flow of oil through the annular oil passage 38 which removes most of the heat which gets into the bearing housing 30. An air cooling system includes an air passage 90′ through the center of the shaft 16 and an air pump 94 that delivers air through the air passage 90′ to move heat out of the shaft 16 in a direction opposite the direction in which heat is soaking into the shaft 16 from a turbine inlet scroll 18.
Similarly, U.S. Pat. No. 6,609,375 discloses an electrically assisted turbocharger 10 including a compressor 16, a turbine 18, and a motor housing 20 therebetween. The turbocharger 10 also includes an electric motor having a stator 42 and rotor 44 which are housed within the motor housing 20. An air cooling system includes a first cooling hose 34 which directs pressurized air from the compressor 16 into the motor housing 20 through an airflow inlet 40. The air travels in a radial direction across the stator 42 and rotor 44, through the motor housing 20, and out an airflow outlet 46 located on an opposite circumferential side of the motor housing 20 from the airflow inlet 40. A second cooling hose 36 directs the air back to an inlet of the compressor 16.
Known air cooling systems art are not optimal in terms of the ability to supplement a liquid cooling system in an electrically assisted turbocharger. It is desirable, therefore, to provide an air cooling system which effectively supplements a liquid cooling system in an electrically assisted turbocharger.